f)0 PROCEEDINGS OF THE NATIONAL MVSEU2I. vol. 37. 



collection of Amherst College, This drawing, here reproduced (text 

 fig. 6) , represents a section across the sheath immediately in front of 

 the eighth tooth, now missing. For comparison with it is shown Kar- 

 pinsky's section of his specimen (text fig. 5). The difference will 

 be readily observed. It may be noted here that Mr. E. T. Newton 

 published « a description of a species of Edestiis, E. triserratus^ 

 found in the coal measures of Britain. The shaft appears to have 

 been much curved. Beneath the tooth the lower border is thin and 



angular. In front of the tooth the border is 



broadly rounded. 



2. ON THE HISTOLOGY OF EDESTUS. 



The organ called Edestus^ whatever its position 

 FIG..J.—EDESTLS MINUS- aud Its functiou, is composed of dentine which 

 cuLus, x3. TYPE. -^^ penetrated bv numerous terminal arteries and 



SECTION OF SHEATH }- 



JUST IX FRONT OF veius aud capillary canals. On the surface of 

 '^°"'^"' what are called teeth is a thin layer of what is 



probably true enamel. The dentine must be classed with that called 

 by Tomes vasodentine, although, like osteodentine, there existed no 

 distinct pulp. Some sections have been made, in order to show the 

 minute structure of the organ in question. These have been prepared 

 from two specimens of what are regarded as Edestiis heinrirhii, 

 which have been most kindly sent me by Dr. Bashford Dean, of the 

 American Museum of Natural History, New York. The specimens 

 came originally from western Indiana. There is rejjresented in fig. 

 1, pi. 13, a section across one of the segments, so taken as to include 

 the front of the tooth. That part which belongs 

 to the tooth broke away from the part below it 

 during mounting. An examination of this figure 

 shows that in this genus the central core of 

 dentine, which contains the larger vessels, was 

 not sharply marked off from the- more suioer- 



n • ^ i.-^ -i. • ^ 1 IT • rr T - Fiu.6. — Edestus minor, 



ncial portions, as it is marked oii m Helicopnon^ ^j ^^^^ sec- 

 as shown by Karpinsky. Below the center of tion of sheath just 

 the section there is a large vessel that probably '^ front m 

 corresponds to Karpinsky 's " Liingscanal." The section appears 

 to have fallen where the canal was sendinp- off a large branch. In 

 the specimen figured all the larger vessels and many of the smaller 

 ones apparently have the lumen ojoen. They are really filled with a 

 transparent mineral, probably calcite. Each, however, has a nar- 

 row black border which represents a deposit of pyrite or marcasite. 

 Many of the capillary channels aj^pear to be filled with pyrite, 

 but this appearance seems often to be due to the position of 

 the vessel in the section, for when the near and the distal walls 



"Quart. Jour. (4e(>l., vol. GO, 19()4, pp. 1^. pi. 1. 



